Update preamble to indicate final status

Author: maurelian

EIPS/eip-721.md

@@ -1,7 +1,7 @@
 ## Preamble
 
 ```
-EIP: <to be assigned>
+EIP: 721
 Title: ERC-721 Non-Fungible Token Standard
 Author: William Entriken <github.com@phor.net>, Dieter Shirley <dete@axiomzen.co>, Jacob Evans <jacob@dekz.net>, Nastassia Sachs <nastassia.sachs@protonmail.com>
 Type: Standard